The embodiments described herein relate to reduction of hydrate inhibitor concentrations in produced water.
Gas hydrates are a growing concern in oil or gas production, at least in part, because gas hydrates can present flow assurance problems in onshore wells, offshore wells, and pipelines. Gas hydrates are a common form of a unique class of chemical compounds known as clathrates, in which a rigid, open network of bonded host molecules enclose, without direct chemical bonding, appropriately sized guest molecules of another substance. In the case of gas hydrates, water acts as the host molecule, enclosing gas molecules such as methane, thereby yielding ice-like crystals of gas and water.
Gas hydrates normally are found in cold climates, in deepwater environments, or at any point in a gas system where the gas experiences rapid expansion. As this lattice expands and gains mass, it can block tubings, flow lines, pipelines, or any conduit through which produced gas flows such as a drill string or a blowout preventer.
As deepwater drilling and production increases, the problems associated with hydrate formation may increase. Deepwater is an ideal breeding ground for the growth of gas hydrates, and when these ice-like crystals form in the circulating system, attempts to manage them can be costly and dangerous.
Hydrate inhibitors are a class of chemicals that inhibit the formation of gas hydrates either kinetically or thermodynamically. In some instances, hydrate inhibitors may be injected into a wellbore and the surrounding formation to mitigate gas hydrate formation in the produced water during petroleum hydrocarbon production. At the well site, the produced water may be separated from the petroleum hydrocarbons (e.g., crude oil and natural gas). Depending on the purity, the produced water may be released to the local environment (e.g., discarded overboard for an offshore rig). However, in some instances, the concentration of hydrate inhibitors and other organic material (e.g., drilling fluid additives and any remaining petroleum hydrocarbons that were not separated from the water like hexane, benzene, and naphthalene) is sufficiently high that this is not an option.
In some instances, chemical coagulants may be used to consolidate the organic material including hydrate inhibitors into a form that can be removed by hydrocyclones, thereby reducing the concentration of hydrate inhibitors in the produced water. However, in some instances, the chemical coagulant may be unable to coagulate and remove a sufficient amount of organic material for disposal of the produced water. This issue may be especially prevalent for deepwater, offshore wells where very high concentrations of hydrate inhibitors may be used because of the low temperatures at the sea floor.
Further, in many instances, this step of consolidating the hydrate inhibitor may be rate limiting to the entire production operation. That is, with limited storage on an offshore rig, the rate of removing the hydrate inhibitors may limit the rate at which the water and, consequently, the petroleum hydrocarbons may be produced. In some extreme instances, the well may need to be shut in for a period of time to allow for the produced water to be sufficiently cleaned.